1. Field of the Invention
The present invention relates generally to the mounting and connecting of electronic devices and, more particularly, to apparatus and methods providing an improved solder ball pad structure on a substrate such as a printed circuit board (xe2x80x9cPCBxe2x80x9d) or a semiconductor die.
2. State of the Art
An increasing demand for electronic equipment that is smaller, lighter, and more compact has resulted in a concomitant demand for semiconductor packages that have smaller outlines and mounting areas or xe2x80x9cfootprints.xe2x80x9d
One response to this demand has been the development of the so-called xe2x80x9cflip-chipxe2x80x9d method of attachment and connection of semiconductor chips to substrates. Sometimes referred to as the xe2x80x9cControlled Collapse Chip Connection,xe2x80x9d or xe2x80x9cC4,xe2x80x9d method, the technique involves forming balls of a conductive metal, e.g., solder or gold, on input/output connection pads on the active surface of the chip, then inverting, or xe2x80x9cflippingxe2x80x9d the chip upside down, and xe2x80x9creflowingxe2x80x9d the conductive balls, i.e., heating them to the melting point, to fuse them to corresponding connection pads on a substrate.
Another response has been the development of a so-called ball grid array (xe2x80x9cBGAxe2x80x9d) semiconductor package that xe2x80x9csurface mountsxe2x80x9d and electrically connects to an associated carrier substrate, e.g., a printed circuit board (xe2x80x9cPCBxe2x80x9d), with a plurality of solder balls in a method sometimes referred to as the xe2x80x9cC5xe2x80x9d method that is analogous to the flip-chip method described above for mounting and connecting dies.
In both the C4 die and C5 package mounting and connection methods, a plurality of solder balls is attached to respective solder ball mounting lands, or pads, defined on a surface of the die or interposer substrate. The solder ball mounting pad may be defined by an opening in an insulative layer or mask called a xe2x80x9cpassivation layerxe2x80x9d in the case of a semiconductor die, or a xe2x80x9csolder maskxe2x80x9d in the case of an interposer substrate of a BGA package, as described below. The interposer substrate in a BGA package may comprise a rigid or flexible sheet material.
In a solder-mask-defined (xe2x80x9cSMDxe2x80x9d) solder ball pad, an aperture formed in the mask over a terminal pad defines the solder ball pad mounting area. Typically, the terminal pad comprises a layer of metal, e.g., copper, aluminum, gold, silver, nickel, tin, platinum, or a multilayer combination of the aforementioned that has been laminated and/or plated on a surface of the substrate sheet and then patterned using known photolithography techniques. Further, one or more circuit traces may be formed simultaneously with the terminal pads using the same processes. In addition, a plated through-hole, called a xe2x80x9cvia,xe2x80x9d may also be formed and may connect the pad layer with the opposite surface of the substrate sheet.
A solder mask is then formed over the metal terminal pad and may comprise an acrylic or a polyimide plastic or, alternatively, an epoxy resin that is silk screened, spin-coated or applied as a preformed film on the substrate sheet. An aperture is formed in the solder mask to expose a portion of the terminal pad, but not any portion of the surrounding substrate surface. A solder ball may be attached to or formed on the terminal pad area thus exposed; however, the solder mask prevents the solder of the solder ball from attaching to any portion of the terminal pad other than the mounting area that is exposed through the aperture. Thus, the exposed area is referred to as an SMD-type of solder ball mounting pad.
Comparatively, a nonsolder-mask-defined (xe2x80x9cNSMDxe2x80x9d) solder ball mounting pad may be formed in a similar manner, the exception being the size of the aperture in the solder mask. In particular, typically, the NSMD pad exposes the entire terminal pad, at least a portion of the surface of the substrate sheet and, optionally, a portion of an adjacent circuit trace, such that the molten solder of the solder ball can attach to the entire surface and peripheral vertical side surface of the terminal pad thus exposed. Typically, a circular-shaped terminal pad and a portion of a circuit trace are exposed in an NSMD solder ball mounting pad arrangement. The connection area of both the SMD-type and NSMD-type solder ball mounting pads may be coated with a nickel layer and then a gold layer to enhance wettability of solder thereon.
Each of the conventional SMD and the NSMD solder ball mounting pads have some advantages as well as disadvantages associated with it.
Turning to the SMD solder ball pad, it provides relatively good xe2x80x9cend-of-linexe2x80x9d (i.e., at the end of the semiconductor package fabrication line) ball shear resistance because the solder mask overlaps the peripheral edge of the terminal pad proximate to the exposed area defining the solder ball mounting pad and, therefore, resists ripping of the terminal pad from the substrate when mechanical forces act on the solder ball attached thereto. In contrast, the NSMD solder ball pad has a relatively lower end-of-life shear resistance because the solder mask does not cover the peripheral edge of the NSMD terminal pad.
The SMD solder ball pad also affords relatively better control of the lateral (x-y) position of the solder ball on the surface of the substrate than does an NSMD solder ball pad. This is because the lateral position of the solder ball on the substrate may be affected by two factors: 1) the position on the substrate of the centroid of the aperture in the solder mask, if the vertical wall of the aperture interacts (e.g., touches, or electrostatically interacts) with the solder ball, and 2) the position of the centroid of the area of the metal pad layer that is exposed by the opening in the mask, i.e., the area wetted by the molten solder of the solder ball when the latter is attached to the solder ball pad. In both instances, the center of gravity of the solder ball tends to align itself over each of the two respective centroids if both factors apply. As a result, when the centroid of the aperture does not coincide with the centroid of the exposed area of the mounting pad and the vertical wall of the aperture interacts with (e.g. touches) the solder ball, the center of gravity of the solder ball may be positioned approximately half way along a line extending between the two centroids. Since in an SMD solder ball pad the aperture in the solder mask exposes only pad layer metal, the centroid of the aperture and exposed metal pad layer coincide. Thus, so long as the aperture in the solder mask is located within the periphery of the metal pad layer, the lateral tolerances of the SMD solder ball will depend substantially on the lateral positional tolerances on the centroid of the aperture.
However, the shape of the NSMD solder ball pad exposed by the aperture in the solder mask includes a terminal pad portion as well as a portion of the circuit trace. Further, the vertical wall of the aperture may not touch the solder ball. Consequently, the centroid of the NSMD solder ball pad, i.e., of the exposed area of metal, is shifted slightly toward the circuit trace and away from the centroid of the opening, which is typically centered on the terminal pad portion. Hence, the center of gravity of the solder ball will be positioned according to the respective centroids of the NSMD solder ball pad and the circuit trace. Thus, the lateral tolerances on the solder ball on an NSMD solder ball pad may depend not only on the lateral tolerances of the centroid of the aperture, but also the lateral tolerances of the centroid of the exposed metal of the metal pad layer as well. Moreover, even without the presence of a circuit trace, misalignment of the solder ball can still occur in an NSMD pad if the centroid of the exposed pad is not sufficiently aligned with the centroid of the aperture, and thus a vertical sidewall of the aperture interacts with the solder ball.
While the lateral misalignment of a solder ball relative to an opening resulting from this xe2x80x9cshiftxe2x80x9d is relatively small, it should be understood that a C4-mounted die or a C5-mounted semiconductor package can typically have a large number, e.g., up to nine hundred, of such solder balls on its mounting surface, and that accordingly, these slight misalignments in the array of balls can be additive, such that in some cases, the die or package cannot be successfully mounted to an associated mounting surface.
As a further comparison between SMD and NSMD solder ball pads, the solder ball attached to an NSMD solder ball pad attaches to the vertical side surface of the exposed metal of the terminal pad including the circuit trace(s), if any. It is postulated that this side surface attachment and resulting arcuate attachment structure helps to distribute stresses resulting from thermal aging so that the stresses do not concentrate at the interface between the NSMD solder ball pad and the solder ball. Thus, the NSMD may provide an improved resistance to thermal stresses over the SMD solder ball pad, the solder ball/pad interface of which consists of a simple planar interface between the exposed portion of the terminal pad and the solder ball.
U.S. Pat. No. 6,201,305 to Darveaux et al. as well as U.S. Pat. No. 5,872,399 to Lee each describes a solder ball pad structure. More specifically, the Darveaux reference describes an NSMD-type solder ball pad structure wherein a layer of metal on the substrate is formed into a terminal pad, the pad having at least two spokes radiating outwardly from it. The pad structure with spokes is exposed by way of an aperture formed through the solder mask such that the terminal pad and an inner portion of each of the spokes is exposed therethrough, and an outer portion of each of the spokes is covered by the mask. The Lee reference describes a solder ball pad structure having a terminal etching hole as well as a plurality of etching holes at the outer portion of the solder ball pad structure for increasing the contact area for a solder ball.
Another area of interest is the design flexibility in the number of circuit traces that may be operably positioned to run between two adjacent solder ball pads with adequate spacing between the traces and between the traces and the solder ball pads. More specifically, the aforementioned tolerance considerations, as well as the differences in the formation of SMD and NSMD solder ball pads, must be factored in determining the spacing between circuit traces and solder ball pads. Of course, dimensional tolerances as well as parameters required to achieve a robust design limit the ability to position additional circuit traces between solder ball pads for a given solder ball pad design pitch.
In view of the foregoing, a method for fabricating solder ball mounting pads on a substrate and resulting solder ball mounting pads which improve on both types of conventional solder ball pads and eliminates some of their respective disadvantages would be desirable.
The present invention comprises an apparatus and method providing an inventive solder ball pad structure and substrates, electronic device assemblies and systems employing same. The solder ball pad structure of the present invention includes a metal terminal pad that is partially exposed by an aperture in a solder mask layer which does not expose any portion of a surrounding substrate surface. In addition, an optional metallic or conductive polymer interface layer may be formed onto at least a portion of the exposed area of the metal terminal pad and onto at least a portion of the vertical sidewall of the solder mask defining the aperture as well as extending onto the surrounding top horizontal surface of the solder mask. Alternatively, an optional nonconductive polymer interface layer may be formed onto the surrounding top surface of the solder mask. A copper layer comprising an electroless copper seed layer as well as an optional electroplated copper layer may be formed over the solder mask, into the aperture and over the exposed portion of the terminal pad. Nickel and gold layers may be applied by electroplating to the copper layer to enhance the wettability to solder of the resulting pad surface. The solder ball pad structure of the present invention may be termed a ball pad on solder mask or xe2x80x9cBPS.xe2x80x9d
The solder ball pad structure of the present invention provides a variety of advantages. First, the lateral positional tolerance of an attached solder ball is largely determined by the tolerances associated with the formation of the solder mask, similar to the SMD solder ball pad. Additionally, the optional interface layer and subsequent metal layers which may be attached to the metal terminal pad enable the solder ball to attach to the vertical side surface of the aperture in the resulting structure, which configuration may provide enhanced thermal stress distribution in the solder ball connection. Also, the solder ball pad of the present invention also provides increased surface area for solder ball attachment, as well as an indented surface for attachment which may further strengthen the bond between a solder ball and the inventive solder ball pad structure.
As another advantage, the solder ball pad structure of the present invention provides additional, usable lateral space on the substrate between adjacent solder ball pads for circuit traces and circuit trace spacing. Because the layered structure above the metal terminal pad on the substrate increases the area for attachment for the solder ball, in excess of the metal pad layer area that is exposed via the solder mask aperture, the size of the metal terminal pad may be reduced. This enables additional circuit traces or spacing to be employed between adjacent solder ball pads, as desired.
It is also contemplated that the structure of the present invention has utility with discrete conductive elements other than solder, such as conductive or conductor-filled epoxy. Accordingly, the term xe2x80x9csolder ball padxe2x80x9d is exemplary and not limiting of the scope of the present invention.